Underwater acoustic transmitters such as acoustic modems are generally used for underwater communications in oceans, lakes and similar environments where radio frequency devices are not practical. Various types of underwater transmitters in use today are based on magnetostrictive and piezoelectric components, and moving coil elements. These transmitters are typically limited in acoustic bandwidth in comparison to land-based communication systems such as high speed cable Internet and 802.11 wireless communication systems. For example, the bandwidths of most underwater acoustic transmitters are typically less than 20 KHz and in many instances the frequency response varies significantly across the bandwidth. The restricted bandwidth is typically due to the material properties of the mechanical to acoustic transducer components used in the underwater transmitters. In addition, the frequency bands often have center frequencies determined according to the mechanical resonance frequencies of the components. Due to the limited bandwidth, the number of communications channels and the type of communications supported by conventional underwater transmitters are limited.
Current underwater acoustic transmitters have other disadvantages. The magnitude of the acoustic signal generated by an underwater acoustic transmitter limits the length of the communications path. Conventional underwater acoustic transmitters require significant power to establish and maintain communications links. Many underwater transmitters generate spatially broad acoustic signals while other transmitter generating more directional acoustic signals are typically limited in their ability to steer the acoustic signal in a desired direction. Moreover, many acoustic transmitters such as some piezoelectric transmitters employing brittle ceramics have limited mechanical reliability. In addition, many underwater transmitters are large devices and are not readily adapted for mounting to submerged structures such as underwater vessels and the underside of surface vessels.